Here is a rework of my earlier post regarding modifying a M553 to run a 3 way system incorporating a slot tweeter ( 2405 / 077 / 076 ) .
In standard trim this unit only allows a MF / HF crossover point adjustable up to 5kHz. Most of the designs incorporating a slot tweet call for a x/o point of 7.5kHz or higher.
In the pic below the components that are colored are the ones that are of interest.
These are small value capacitors and they are used to set the filter frequency.
The PINK ones are 0.0018 and are the ones that set the filter in the 4 way mode.( 9 kHz maximum )
The GREEN ones are the caps that set the MF/HF point in stereo mode. ( 5 kHz maximum )
These are 0.0032.
To make the unit work at the higher frequency in stereo mode we need to change all the GREEN caps to 0.0018.
This is a standard cap value and these little guys cost pennies but be warned,
the solder tracks are very small and if you are in any way doubtful of your soldering skills have someone else do it for you .
Hope this helps anyone looking to go down this road. Mine is working a treat.